The present invention concerns a new stopper rod for regulating the flow of a molten metal from a pouring group, for example, steel or cast iron, from a distributor or a casting ladle, and more particularly a one-piece stopper rod that has means for attachment to a lifting mechanism. In a particular embodiment, the stopper rod has also means for introducing an inert gas, such as argon, into the molten metal bath during continuous casting operations.
Such a stopper rod and its use are well known to the skilled artisan, in particular by U.S. Pat. Nos. 4,946,083 and 5,024,422 to which reference is made. Among others, these documents describe a one-piece stopper rod that can be attached to a lifting mechanism, comprised of:
a) an elongated body of refractory material that has a borehole positioned coaxialy with respect to the body of the stopper rod and adapted to receive fixedly a metal rod for its attachment to a lifting mechanism. The axial borehole of the body of refractory material has an enlarged part with an annular sealing surface spaced away from the upper end of the body of refractory material. The means for attachment of the metal rod are generally positioned between the enlarged part and the lower end of the body of refractory material. At its lower end, the body of refractory material may have means for introducing gas into the molten metal bath; and
b) an elongated metal rod attached to the body of refractory material and having an axial borehole communicating in its lower part with the borehole of the body of refractory material. The rod has a collar carrying an annular sealing surface facing the annular sealing surface of the borehole of the body of refractory material to create a gas tight seal. The upper end of the rod is adapted to be attached to a lifting mechanism that permits displacing the stopper rod vertically inside of a pouring group such as a distributor.
The stopper rod is connected to a gas supply line, generally but not obligatorily, through the upper end of the rod. Means of attachment to the body of refractory material are generally positioned between the collar and the lower end of the metal rod. During the use of such a stopper rod, the gas introduced into the stopper rod is conveyed toward the axial borehole of the body of refractory material in its lower part. Thanks to the means for introducing gas into the molten metal bath that the body of refractory material has in its lower part, the stopper rod permits the introduction of gas into the molten metal bath. The facing annular sealing surfaces of the rod and the body of refractory material prevent significant losses of inert gas as well as the infiltration of air.
To improve this tightness even more, it was proposed to place a gas tight annular gasket between these sealing surfaces. The U.S. Pat. No. 4,946,083, for example, indicates that when a gasket with a thickness of ca. 0.4 mm and of material resistant to high temperatures, e.g., graphite, is in place, the interface between the annular sea surfaces of the rod and the body of refractory material furnishes tightness capable of resisting a pressure up to 3 bars.
The German patent DE-C1-4040189 discloses a stopper comprising an elongated refractory body having
a) an axial borehole and means (locking pin) for attaching a metal rod,
b) a metal rod and
c) a sleeve like body compressing a sealing gasket under actuation of a nut.
With the device of this document, it is necessary to correct or fly tune the tightness when the mounting rod is fixed in the stopper body by screwing the nut against the upper end of the stopper. The system is not self-tightening and requires human intervention (screwing the nut) when the stopper has reached the final temperature. Such an intervention above the molten metal bath is uneasy and extremely dangerous. The seal is essential for casting high-grade molten metal. In the first place, it is necessary to assure a good protection against the filtration of air responsible for oxidizing the molten metal during pouring. On the other hand, it is also indispensable to minimize the losses of inert gas (in case an inert gas is injected through the stopper rod) that cause production cost overruns that are far from negligible. Beside this tightness issue, it is also essential that the attachment of the stopper rod to the mechanism remains as rig as possible.
The system in use at the present time still do not however furnish a completely satisfactory solution for these two points of view.
In conducting this research in this domain, the applicant discovered that these problems are due to the fact that Sealing gasket between the annular sealing surfaces of the rod and the body of refractory material facing each other does not remain compressed during the entire pouring operation.
It is believed that this loss of compression in the sealing gasket is due, at least partially, to the difference between the coefficients of thermal expansion of the different materials constituting the stopper rod. In particular, under the effect of the temperature to which the stopper rod is raised during pouring, the metal rod becomes highly dilated relative to the body of refractory material. This more substantial expansion of the metal rod has the effect of separating the annular sealing surfaces of the rod and the body of refractory material and consequently reducing the compression of the sealing gasket with all the adverse consequences involved.
According to the prevent invention, this problem is remedied by giving the stopper rod particular means for maintaining the compression of the sealing gasket in contact with the annular sealing sure of the body of refractory material when the stopper rod is brought to a high temperature. The stopper rod involved in the present patent application is, notwithstanding the means for maintaining the compression of the sealing gasket, similar to that described in U.S. Pat. Nos. 4,946,083 and 5,024,422 to which reference is made.
According to the invention, the means of maintaining the compression of the sealing surface when the stopper rod is brought to high temperature is furnished by a sleeve presenting the form of a cylinder open at its ends, which is fitted on the metal rod. The present invention concerns a one-piece stopper rod that can be attached to a lifting mechanism, comprised of:
an elongated body of refractory material, having
(i) a borehole positioned coaxially with respect to the body and adapted to receive fixedly a metal rod for its attachment to a lifting mechanism, the axial borehole having an enlarged part that presents an annular sealing surface spaced away from the upper end of the body;
(ii) means for attaching the said metal rod;
an elongated metal rod fastened to the body adapted at its upper end to be attached to a lifting mechanism for vertically displacing the stopper rod inside of a pouring group; and
a sleeve having a sealing surface at its lower end facing the sealing surface of the body, the stopper rod having means located on the metal rod for blocking the sleeve, the stopper being characterized in that the sleeve is formed of a material having a coefficient of thermal expansion greater than that of the metal rod and having a sufficient length so that, under the effect of the temperature to which the stopper rod is brought during pouring, the sleeve expand sufficiently to at least compensate for the effect of expansion of the metal rod.
According to a particular embodiment of the invention, the stopper rod can be connected to a gas supply line. Therefore, said elongated body of refractory material has, at its lower end, means for introducing gas into the molten metal bath and said metal rod has an axial borehole communicating in its lower part with the borehole of the body of refractory material.